Hydraulic pumping assemblies of the type including a swash plate and a plurality of axially orientated pistons are well suited for use in the steering system of a marine craft. More particularly, the movements of an outboard motor or rudder of the marine craft are often controlled by a hydraulic cylinder reacting to the pressure of fluid in a hydraulic steering system. Typically, a steering wheel is attached to an input shaft of the hydraulic pumping assembly, which steering wheel is turned by an operator to control the course of the craft.
It is common practice to provide a central hollowed cavity in pistons for a number of different reasons. One reason for providing a cavity in the pistons is to receive a return compression spring for urging the pistons out of their associated chamber and against the swash plate camming surface. The springs will usually nest, or telescope, into the piston cavity with the reciprocating movement of the piston. A typical pump mounting orientation in the marine craft supports the pistons in an inverted position wherein they are urged by the return springs in a generally upward direction out of their associated chambers. This mounting orientation is quite practical and facilitates connection of the conduit lines behind the dashboard of the marine craft.
A persistent problem results from orientating the pump as described above in that gas inclusions trapped in the hydraulic fluid, upon entering the cylinder chamber, move toward the highest elevational position. This highest elevational position is frequently adjacent a top surface in the cavity of the piston. Therefore, during operation of the prior art hydraulic swashplate type pumping assemblies, air inclusions tend to congregate adjacent this top spaced surface in the cavity of the piston and resist discharge with the fluid when the piston is moved into a compressed position. Failure to remove these trapped gas inclusions from the cylinder chamber result in lost pumping efficiency due to the additional compression required to compress the air prior to generating enough pressure in the hydraulic fluid to do work. Additionally, in a closed circuit conduit system, fluid is not moved through the conduit system while the pistons compress the gas inclusions.
The U.S. Pat. Nos. 3,935,796 to Wood, issued Feb. 3, 1976, and Reissue 24,048 to Wright, issued Aug. 2, 1955, disclose examples of the prior art described above wherein the pistons of a swash plate pump include a central hollow cavity for receiving a return compression spring. The U.S. Pat. No. 3,280,395 to Budzich, issued Sept. 28, 1965, discloses a swash plate hydraulic pump assembly including axial pistons having central hollow cavities. The Budzich piston cavities are provided for reasons other than receiving a return spring therein. Accordingly, when any one of these prior art hydraulic pumping assemblies are operated in such an orientation that the pistons are urged in an upward direction out of their associated chambers, gas inclusions trapped in the hydraulic fluid entering the chamber will migrate toward and remain adjacent the uppermost spaced surface, and resist expulsion with the discharging fluid.